This invention relates to steam generators and, more particularly, to an apparatus for facilitating removal of sludge deposits from the tubesheets of steam generators.
A typical nuclear steam generator generally comprises: a vertically oriented shell; a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle; a tubesheet for supporting the tubes at the ends opposite the U-like curvature; a plurality of tube support plates spaced along the tube bundle parallel and above the tubesheet; a plurality of cylindrical spacers extending perpendicular between adjacent tube support plates; and a plurality of stayrods, each having a diameter larger than each tube, extending through the tube support plates and spacers and being threaded into the tubesheet to limit deflection of the tube support plates during break loadings of the steamline and feedline of the steam generator.
The steam generator also comprises a wrapper barrel disposed between the tube bundle and the shell to form an annular chamber adjacent the shell, and a feedwater header or ring disposed above the U-like curvature end of the tube bundle. Primary fluid having been heated by circulation through the reactor core enters the steam generator and moves through the U-tube bundle. While feedwater is circulating in heat transfer relationship around the tube bundle, heat is transferred from the primary fluid circulating in the tubes to the feedwater plus recirculating water, thus causing a portion of the recirculating water to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment, thereby generating electricity in a manner well known in the art.
Such nuclear steam generators must be serviced during shutdown periods to remove sludge left from the feedwater which predictably accumulates at the tubesheet. Servicing involves opening handhole covers near the tubesheet region, and inserting a sludge lancing tool therein, such as described in U.S. Pat. No. 4,276,856, entitled Steam Generator Sludge Lancing Method," issued to Dent et al. and assigned to Westinghouse Electric Corp. The sludge lancing tool shoots a jet of high pressure water into the respective lanes formed between the tubes, thus loosening the sludge from the tubesheet surface, and entraining the sludge in a flow of water which is then drawn off of the tubesheet. The stayrods, however, have been known to impede lancing of some areas of the tubesheet since they are larger than the tubes and effectively block some of the sludge lancing lanes.
In light of the above problem, more efficient sludge lancing of the tubesheet is required.